In recent years, as eating habits have changed dramatically, excessive weight gain caused by the increasing amounts of sugar found in foods have resulted in health related problems. Accordingly, the development of a low-calorie sweetener (sweetening agent) that replaces sugar has been strongly in demand. An example of such a low-calorie sweetener that is commonly used is aspartame, which is safe and effective for providing a high level of sweetness. However, aspartame is somewhat unstable.
To solve these problems, an -[N-[3-(3-hydroxy-4-methoxyphenyl)-3-methylbutyl]-L-α-aspartyl]-L-phenylalanine 1-methyl ester shown below has been found, which is not only highly stable but is also far better with respect to the sweetening potency it imparts. 
To make the N-[N-[3-(3-hydroxy-4-methoxyphenyl)-3-methylbutyl]-L-α-aspartyl]-L-phenylalanine 1-methyl ester a process of reductively alkylating a β-O-benzyl-α-L-aspartyl-L-phenylalanine methyl ester with a 3-(3-benzyloxy-4-methoxyphenyl)-3-methylbutyl aldehyde followed by removing the benzyl group of a protecting group therefrom has been provided previously by the present inventors. However, in this process, the aldehyde, which is used as an intermediate in the process, requires 7 reaction steps to be synthesized from the 3-hydroxy-4-methoxy acetophenone, which is used as the starting material. This 7-step reaction scheme is shown in the Reaction Process 1 below. Therefore, from the point of industrial profitability, such a reaction is not desirable. 
In view of this problem, there is a need in the art for a process for producing industrially and easily the aspartyl dipeptide ester derivative described above.
Therefore, to solve this problem, the present inventors set out to provide processes for industrially and efficiently producing the N-[N-[3-(3-hydroxy-4-methoxyphenyl)-3-methylbutyl]-L-α-aspartyl]-L-phenylalanine 1-methyl ester; and novel intermediate compounds useful in such production processes.